1. Field of the Invention
This invention relates to a technique for improving weighing accuracy of a load cell which is used for indeterminate quantity weighing or determinate quantity weighing of granular goods or so in the distribution processes, etc.
2. Description of the Prior Art
As is known, various parts, goods, etc. are weighed in many industrial fields. For example, indeterminate quantity weighing is made for various parts in the manufacturing sections and determinate quantity weighing is made for many goods, in addition to indeterminate quantity weighing, in the distribution sections in order that weight of goods will not be insufficient with respect to the preset weight and an excessive amount will become minimum.
In either weighing practice, it is essential for these years to make weighing in an accurate, repeated and rapid manner, and weight of articles is determined based on the relative relationship between a loaded state of the articles and a weighing machine. It is practically possible to select a weighing machine which has accuracy necessary for weighing to be conducted at minimum, because the machine is an industrial product, but it is difficult to promptly grasp a loaded state of articles as a uniform state for regular weighing, because such a loaded state is very irregular depending on, e.g., the dropped or thrown-in condition of articles.
It was customary that the loaded state of articles to be weighed was neglected to some degree and such irregularities were corrected through calibration of the weighing machine.
For instance, a weighing machine disclosed in Japanese Utility Model Laid-Open No. 54-171368 is constructed as follows, as shown in attached FIG. 1. A weighing machine 1 has a frame 2 to which a pair of leaf springs 3, 3 are attached in parallel to each other, and a rebound leaf spring 4 is interposed therebetween. A bracket 5 as a support is attached to the distal end of those springs, and a load cell 7 is supported on the bracket pivotably in the horizontal direction through a buffer rubber 6 interposed therebetween. To the distal end of the load cell 7 there is attached through another buffer rubber 6 another support 8 pivotably, on the upper part of which is provided a receiving saucer 9 as a weighing member.
In such prior art weighing machine 1, as seen from the technique shown in said Laid-Open print, to avoid the occurrence of permanent strain, etc. in the load cell 7 when a thowing-in shock is applied to the receiving saucer 9 with the weighed material to be put thereon, a lateral shock is absorbed by the rubbers 6 to turn the load cell 7 in horizontal direction and a vertical shock is absorbed by the pair of springs 3, 3.
Another leaf spring 4 is arranged to press the upper leaf spring 3 thereby to vibrations of the leaf spring 3, but it is apparent that the spring 4 has substantially only a buffer function.
Probably, the reason is in that the leaf spring 4 produces friction resistance against the leaf spring 3 through pressing, but has no anti-vibration function due to elastic resistance.
In view of the above, an experiment model was manufactured for the conventional weighing machine as shown in FIG. 1 and measurement was conducted in such a state that the load was taken away in a moment. The result is shown in FIG. 2 in which the axis of abscissas represents time t and the axis of ordinates represents an amplitude a. As seen from FIG. 2, a damping characteristic is very inferior and vibrations will not be damped promptly, thereby leading to the disadvantage that a weighing signal from the load cell 7 will not exhibit the accurate weight value within a desired given time after throwing-in or loading of the materials to be weighed and a time characteristic for achieving stable weighing will become inferior.
It is, therefore, concluded from the above including the data shown in FIG. 2 that the conventional weighing machine 1 has not the anti-vibration effect substantially but just a buffer function alone.
To cope with this, weighing was made in the past with a filter of an analog circuit being set to have lower cut-off frequency as an expedient manner, thereby to cut the signal of higher frequency substantially as shown in FIG. 3. Measurement was started after the damping characteristic comes into a practically stabilized state with the lapse of a certain period.
Also as shown in FIG. 3, however, there exist in practice minute vibrations (later described) of lower frequency that are propagated from the base side and can not be eliminated by the filter. This was another reason of making it difficult to achive high-accurate weighing.
More specifically, a weighing line which requires weighing of high accuracy includes the following disadvantage. A base on which the weighing machine is installed undergoes the so-called complex base vibrations attributable to the associated the ground, building and mount, other weighing lines, the concerned line itself, etc. Those vibrations are transmitted and applied to the load cell, so that the load cell can not detect the weight in a stable manner.
In this way, despite it is essentially desired for a load cell incorporated in a weighing machine that vibrations are promptly damped when the materials to be weighed have been loaded by throwing-in or other steps, and that even if various base vibrations due to disturbance are present, these are eliminated and stabilized as soon as possible to permit immediate detection of proper weight, as shown in FIG. 4, the conventional weighing machine as mentioned above could not meet such conditions. Stated otherwise, the prior art weighing machine had a fatal defect that it can neither provide the anti-vibration effect, nor eliminate base vibrations, and it had just a buffer function of avoiding the occurrence of permanent strain.
To cope with this, there has been further proposed, for example, a method in which an additional dashpot or so is provided to absorb vibration energy with viscosity resistance for the purpose of damping disturbance vibrations promptly. In a transducer of such mechanism as including a load cell which undergoes very minute displacement and detecting the load based on deformation of a strain generator, however, design and manufacturing are very complicated which must be suitable in points of a displacement amount of the load cell, a displacement rate thereof and absorption energy so as to achive the sufficient effect by use of a damper, e.g., a dashpot, provided on the outside while keeping the desired weighing accuracy and sensitivity. This results in a problem that the foregoing dash pot or other dampers can not be installed in practice.
As previously noted, the conventional weighing machine also has the disadvantage that lateral vibrations are damped by the rubbers 6, but horizontal vibrations can not be restrained, thereby resulting in three-dimensional vibrations which impart a complex vibration phenomenon.